As telephone switching systems are required to work non-stop, 24 hours a day, they must be made of the most reliable component arranged in the most efficient manner possible. But the use of highly reliable components alone is not enough to guarantee that breakdowns will not occur. Various ways of achieving the high reliability and maintainability of telecommunication switching systems have been in existence. These are the dual synchronous matched scheme, dual load sharing scheme, triplicated system, multi-computer hierarchy system, multi-computer democracy system, and multi-processor system, to name a few. Among these schemes, the dual synchronous matched scheme (dual plane operation) has been proven very successful.
While this scheme is fairly routine in TDM switching, it has not been adapted for message switching or computing except in specialized cases. The reason for this is primarily the difficulty of macroscopic (message) synchronisation between the duplicated systems. The load-sharing/standby approach in these systems is also justified on the basis of end-to-end recovery protocols.
The ATM switching operation possesses attributes of both synchronous and asynchronous behaviour. The generally synchronous nature of ATM transport should blend into the dual plane operation scheme nicely. What must still be dealt with, however, is the asynchronous behaviour introduced by queues in the switches and multiplexers.
In TDM switches, disagreement between the planes can be dealt with channel by channel. The faulty channel generally does not affect other channels, their order or contents, or their connections because every channel is attached to a hardware timeslot. This is not the case for ATM switches. Since cells are not attached to timeslots but freely compete through arbitration mechanisms for available timeslots, any address change or other interference with the arbitration mechanism, for example as a result of a bit error, can change the order of cells. Consequently, a receiving peripheral cannot expect to receive matching pairs of cells from the two planes in the case of such errors. Yet the purpose of the dual plane operation is to allow peripherals to select one stream of cells from the signals received from each plane, preferably cell by cell.
A detailed description of the typical dual plane operation in the conventional digital switching environment is found in an article entitled "The DMS-100 Distributed Control System" by Bourne et al in Telesis, No. 4, 1980, pp 6-12.
The present invention addresses difficulties associated with the above-mentioned dual plane operation of ATM switches and proposes certain solutions.